the different Cryftalline Forms. 401 
round the cryftal till he is oppofite the angle on which the 
decrements he is confidering take place ; or, what amounts 
to the fame thing, that he is fuppofed to turn the cryftal 
until the angle in queftion is found oppofite to him ; and it 
is in regard to’this pofition that fuch a decrement is faid to 
take place towards his right or towards his left. For ex- 
ample, if the angle A be under confideration, we muft con- 
ceive that the obferyer, who at firft was oppofite to the point 
O, has placed himfelf oppofite to A; then, by full fuppofing 
figure 3 to reprefent the fame folid as figure 1, the decre- 
ments to the right will be thofe ‘which take place on the 
plane AEs-z, parallel to the diagonal drawn from E tor; 
and the decrements to the left will take place on the plane 
Alur, parallel to the diagonal which goes from T to 7. 
We fhall fee hereafter the advantage of viewing in this 
manner, in regard to the uniformity of method. 
g. To denote the firft of the three decrements of which 
I have fpoken, or that which takes place on the bafe P, the 
indicating figure muft be placed above the letter. To de- 
note the fecond, or that which takes place towards’ the 
right, the figure muft have the ufual place of an exponent 
en the right, and towards the top of the letter; and the 
third, or that which takes place towards the left, muft be 
denoted by placing the figure towards the left, and, in the 
like manner, towards the top of the letter, 
Thus O will exprefs the effect of a decrement by two 
ranges in breadth, parallel to the diagonal of the plane P, 
which paffes through the angle E (fig. 1); O° the effect of 4 
decrement by three ranges, parallel to the diagonal of the 
face T, which paffes through the angle I; and *O the effect 
of a decrement by four ranges parallel to the diagonal of the 
face M, which paffes through the angle E. 
10. In regard to the decrements on the edges, thofe 
- which take place towards the contour BC FD of the bafe 
may be expreffed by a figure placed above or below the let- 
Vou, II. Dd ter, 
